Many existing digital wireless or mobile telephone networks make use of time division multiple access (TDMA) to share out radio resources between a number of mobile stations and between a number of channels. For example, in the European Telecommunications Standards Institute (ETSI) GSM standard, a given frequency band is divided in the time domain into a succession of frames, known as TDMA (Time Division Multiplexed Access) frames. The length of a TDMA frame is 4.615 ms. Each TDMA frame is in turn divided into eight consecutive slots of equal duration. In the conventional circuit switched transmission mode, when a call is initiated, a full rate bidirectional traffic channel (TCH/F) is defined for that call by reserving two time slots (1 to 8), in each of a succession of TDMA frames, for the duration of the call. One of these slots provides the downlink from the base station (BS) to the mobile station MS) whilst the other provides the uplink.
The circuit switched transmission mode in GSM provides for a data transmission rate of 9.6 kbps. However, due to the demand for higher transmission rates, a set of GSM enhancements known as GSM Phase 2+ have been specified by ETSI. One of the main features of GSM Phase 2+ is known as High Speed Circuit Switched Data (HSCSD—specified in GSM 02.34 and GSM 03.34) which achieves an increased data transmission rate by using more than one TCH/F for a single connection (i.e. effectively reserving two or more consecutive time slots in each TDMA frame).
GSM Phase 2+ also specifies (see for example GSM 01.60, 02.60, 03.60, and 03.64) a new feature known as General Packet Radio Service (GPRS). GPRS provides for the dynamic allocation of radio resources, with the allocation for uplink and downlink communications being made separately and independently of each other. That is to say that a time slot is allocated to a particular MS to BSS link only when there is data to be transmitted. The unnecessary reservation of a TCH/F, when there is no data to be transmitted, is thus avoided. In addition, a high speed packet switched transmission channel may be provided by assigning two or more slots in each of a succession of TDMA frames to a single MS.
In the current GSM standard, because only a single time slot in each TDMA frame can be reserved for the uplink channel, and similarly for the downlink channel, it is easy to keep the two reserved slots separated in time so that a single radio module can be used, in the MS, for both transmission and reception. This module can also be used for monitoring the radio conditions in the serving cell and in neighbouring cells. However, with the introduction of HSCSD and GPRS where the number of reserved slots in a TDMA frame is variable, if slots are reserved for both uplink and downlink transmission in the same TDMA frame then there exists the possibility that uplink and downlink slots will overlap in time. Communication is then ‘full-duplex’ and it is necessary to provide separate radio modules for transmission, reception, and monitoring, i.e. a total of three radio modules. The preferred option is therefore to use only ‘half-duplex’ communication where uplink and downlink transmissions are made in alternate TDMA frames. The possibility for uplink and downlink transmissions to overlap is therefore eliminated as is the need for separate radio modules in the MS. Current proposals are for symmetric uplink and downlink resource allocation where the same number of time slots in alternate frames are reserved for both uplink and downlink transmissions.